1. Field of the Invention
This invention relates to a device and method for recording a refractive index pattern in an optical medium and has particular application to forming a refractive index grating in an optical waveguide such as an optical fibre.
2. Related Art
It is known that the refractive index of an optical fibre can be altered by exposing it to high intensity light. Germanium doped fibre exhibits photosensitivity in this manner, particularly in response to ultraviolet (u.v.) radiation, and the effect can be used to form a so-called refractive index grating in the fibre. Reference is directed to K. O. Hill et al, "Photosensitivity in Optical Waveguides: Application to Reflection Filter Fabrication" Applied Physics Letters Vol. 32, No. 10 647 (1978). The grating can be formed by producing an optical interference pattern with two interfering beams, and exposing the optical fibre to the interference pattern, so as to record the pattern in the fibre. The interference pattern can be formed by directing an optical beam longitudinally through the fibre and reflecting it back along its path through the fibre, so as to produce a standing wave pattern, which becomes recorded in the fibre due to its photosensitivity. This method is difficult to control in practice and there is a limit on the length of fibre that can be exposed in this way.
In an alternative method, beams derived from a coherent source such as a laser are directed transversely of the length of the fibre, so as to interfere with one another and produce an interference pattern externally of the fibre, which becomes recorded in the fibre as a result of its photosensitivity. A block for producing an external interference pattern for this purpose is described in EP-A-0 523 084.
Another way of forming the grating is to use a phase mask in which the desired amplitude pattern has been recorded holographically as a mask pattern. The phase mask is placed adjacent to the fibre and illuminated with laser light so as to expose the fibre to the holographic pattern. Reference is directed to K. O. Hill et al "Bragg Grating Fabricated in Monomode Photosensitive Fibre by u.v. Exposure through a Phase Mask" Applied Physics Letters Vol. 62 No. 10, 1035 (1993), and also to R. Kashyap et al "Light-sensitive optical fibres and planar waveguides", BT Technol. J. Vol 11, No. 2 (1993).
For a general review of refractive index gratings, reference is directed to "Photosensitive Optical Fibres: Devices and Applications" R. Kashyap, Optical Fibre Technology 1, 17-34 (1994).
A problem with the prior techniques is that there is a limit to the length of refractive index grating that can be formed. With the technique described in EP-A-0 523 084, the length of fibre that can be exposed at any one time to the grating pattern, is limited by the width of the block that produces the external interference pattern and the coherence of the beam, and is typically of the order of 1 cm. When a phase mask is used, the holographic pattern is limited primarily by the length of the phase mask and the width of the beam of coherent light used to illuminate the mask. In practice, the width is limited to the order of 1 cm, although longer gratings have been attempted by a repetitive scanning technique as described by J. Martin et al "Novel Writing Technique of Long Highly Reflective in Fibre Gratings and Investigation of the linearly Chirped Component" Proc. Conference on Optical Fibre Communications, OFC '94 post deadline paper PD29-1, 138, 1994.
Refractive index gratings, which operate as Bragg gratings, have many applications in optical data communications systems, as discussed by Kashyap supra, and in particular, may be used as wavelength filters. The bandwidth of the filter is a function of the length of the grating along the fibre and it is therefore desirable to be able to form gratings of extended length. Hitherto, this has proved difficult.